Monarch butterflies migrate incredibly long distances, from the U.S. to Mexico, and scientists have long wondered how they navigate. Now they know the butterflies use the angle and intensity of sunlight to set their internal clock, giving them a precise sense of when it’s time to head South. And the generation that migrates lives much longer than the others.

Dr. Steven M. Reppert did experiments in which Monarchs were subjected to different patterns of light and darkness during the time they were developing from caterpillars to butterflies. The mature Monarchs were attached to a fine wire that allowed them to fly horizontally in any direction they chose. The experiments showed that Monarchs exposed to normal sunlight during their development flew correctly southwest, but butterflies who developed in constant light lacked this navigation skill. Instead, they always flew toward the sun, east in the morning and west in the evening.

The light exposure was altered for some of them so that daylight seemed to start six hours early. This set their internal clock six hours off the actual position of the sun. While they flew toward the southeast, they were more than 100 degrees off the correct migratory path. These experiments show that Monarchs learn how to tell when winter’s coming while they’re still caterpillars.

There are at least four generations of Monarchs each year. The first generation emerges from eggs laid in Texas and Louisiana in the early spring. The larvae feed on the leaves of milkweeds and then spin cocoons and emerge as butterflies. These adult Monarchs don’t head south, they fly further north, following the milkweeds, and breed again.

This continues until the fall, when the last generation of the year’s Monarchs, responding to the changed angle of the sun, starts flying towards Mexico as soon as they emerge from the chrysalises, on a journey that takes them two months.

Millions of Monarchs spend the winter in Mexico; then that same generation flies to the southern United States to mate and die, starting the cycle all over again. The migrating Monarchs live about nine months, while the other generations live for only a few weeks.

The unique and mystifying migration of Monarchs is only one of the anomalies on this mysterious continent.

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